The direction finding technique of radio signals may be roughly divided into a wideband direction finding technology and a narrowband direction finding technology depending on the range of simultaneous finding.
Narrowband direction finding is a single direction finding technology used in a narrow frequency band at which there exists one interest signal and has an excellent accuracy of direction finding. The wideband direction finding is a technology used in a wide frequency band at which a number of narrowband interest signals exist to detect the signals existed in the wideband and has a poor accuracy of direction finding as compared with the narrowband direction finding technology. However, the wideband direction finding is able to cover a wider band in a single system, and thus, is essentially used in a field of automated monitoring of radio waves.
Problem in both of the wideband and narrowband direction finding technologies is a lowered accuracy of the direction finding if signal strength of a target signal is weak or a finding time is shorter compared to a full finding time. Although detailed reasons why the accuracy of the direction findings decreases may be different, the problem may be generally caused by environmental factors that the signals cannot be properly detected or extracted.
In the field of common wireless communications, in order to solve the problem, there has been introduced a method of digital beam-forming signals received by an antenna array to eliminate spatial interference affected on the received signal. This method has an advantage in terms of the direction detection to reduce the spatial interference of the target signal, but still has a technical constraint in the improvement of the basic performance in the direction finding for detecting the direction of a signal source originating from the direction unspecified.